In conventional syringes, including those intended to be disposable after a single usage, it is mechanically possible to re-load and discharge the instrument repeatedly. Such re-use of syringes has been identified as a source for spreading blood-transmitted infectious agents, including hepatitis viruses and the human immunodeficiency virus (HIV), which is believed to play a role in the causation of acquired immunodeficiency syndrome (AIDS).
Various health agencies, including the World Health Organization, have recently expressed an interest in the development of an inexpensive, non-reusable syringe to prevent patient-to-patient HIV transmission, particularly in the third world or other development countries.
While the prior art teaches numerous syringe designs having means for rendering them inoperable after a single use, none of these designs has seen widespread acceptance or use by the medical community. An important reason for such non-acceptance stems from the readily apparent complexity of previously proposed single-use designs. See, for example, Canadian Patent No. 872,929 (Solowey). Such complexity not only adds to the cost of producing the syringes, but also makes them unreliable and detracts from the simplicity of use which the medical community expects and demands. The increased cost factor is especially important in relation to the developing countries, where the spread of HIV is particularly threatening.
Some prior art syringes, in an attempt to achieve non-reusability, enlarge a portion of the inside diameter of the syringe barrel so as to provide a retention means for on the inside of the syringe barrel to be engaged by an engagement member associated with the piston member. In doing this, however, such syringes impair the sealing integrity between the piston member and the internal wall of the syringe barrel.
The majority of the prior art single-use syringe designs require some action to be taken by the medical staff using them to render them inoperable. For example, U.S. Pat. No. 4,790,822 (Haining) requires the user to withdraw the plunger to retract the hypodermic needle into the syringe barrel and to snap off the plunger shaft to prevent the needle from becoming re-exposed. Public health researchers have found that medical staff, particularly in third world countries, are not taking the action necessary to render such prior art syringes inoperative. Accordingly, passive inactivation of the syringe is desirable. That is, to be effective and accepted by the medical community at large, a single-use syringe must be rendered inoperative without requiring any deliberate action (e.g., withdrawal of the plunger and breaking of the hypodermic needle) on the part of the user. The user in such instance is not required to depart from familiar procedures which he/she has become accustomed to over many years.